Added CentOS RPM package building environment

Signed-off-by: Teddysun <i@teddysun.com>

docker/rpmbuild/README.md

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+## CentOS RPM package building environment by Teddysun
+
+This docker image can be used to build RPM packages.
+
+For more information on docker and containerization technologies, refer to [official document][1].
+
+## Supported tags and respective `Dockerfile` links
+
+- `latest`, `8` [*(Dockerfile)*][2]
+- `7` [*(Dockerfile)*][3]
+
+### Reference
+
+- Supported architectures ([*more info*][4]): `amd64`, `arm64v8`
+
+## Integration
+
+RPMs will be built in `/home/builder/rpmbuild` folder, which should contain source archives, patches and built RPM/SRPM files.
+
+## Prepare the host
+
+If you need to install docker by yourself, follow the [official installation guide][5].
+
+## User Account and Root Access
+
+The `builder` user (UID 1000) is a member of users and wheel, and has password-less sudo as any user, any group.
+
+## Pull the image
+
+For CentOS 7
+
+```bash
+$ docker pull teddysun/rpmbuild:7
+```
+
+For CentOS 8
+
+```bash
+$ docker pull teddysun/rpmbuild:8
+```
+
+It can be found at [Docker Hub][6].
+
+## Start a container
+
+There is an example to start a container for CentOS 7 like below:
+
+```bash
+$ mkdir -m 777 -p /opt/builder7
+$ docker run -it --rm --name rpmbuild7 -v /opt/builder7:/home/builder/rpmbuild teddysun/rpmbuild:7
+```
+
+There is an example to start a container for CentOS 8 like below:
+
+```bash
+$ mkdir -m 777 -p /opt/builder8
+$ docker run -it --rm --name rpmbuild8 -v /opt/builder8:/home/builder/rpmbuild teddysun/rpmbuild:8
+```
+
+[1]: https://docs.docker.com/
+[2]: https://github.com/teddysun/across/blob/master/docker/rpmbuild/Dockerfile.centos8
+[3]: https://github.com/teddysun/across/blob/master/docker/rpmbuild/Dockerfile.centos7
+[4]: https://github.com/docker-library/official-images#architectures-other-than-amd64
+[5]: https://docs.docker.com/install/
+[6]: https://hub.docker.com/r/teddysun/rpmbuild/